Research conducted in our clinic and investigations in other university centers clearly indicate that Attention-Deficit Hyperactivity Disorder (ADHD) is a common behavioral concomitant of Tourette syndrome (TS) in child patients. For many individuals, ADHD symptoms are a greater impediment to social and academic success than their tics, particularly during the elementary school years. Unfortunately, the most safe, effective, and widely used pharmacological treatment for ADHD, namely methylphenidate, is widely thought to exacerbate TS symptoms and therefore to be contraindicated in such patients. Because (a) the pharmacological alternatives to methylphenidate are often associated with increased risk of adverse reactions (e.g., neuroleptics) or are less efficacious for extended use (e.g., imipramine) and (b) there is very little research on methylphenidate in TS patients, we believe that a compelling need exists to examine the safety and efficacy of methylphenidate for ADHD and TS symptoms and to develop drug evaluation procedures for clinical application. Further, such research may provide insight into the neurophysiology of TS. For the proposed study, 20 boys between 6 and 12 years of age and diagnosed as having both ADHD and TS will be entered into an eight-week placebo controlled, double-blind crossover study of methylphenidate. Drug conditions will consist of placebo and .1, .3, and .5 mg/kg of methylphenidate administered twice daily, each for two weeks. Medication will be dispenced in dated envelopes, and patients will be randomly assigned to counterbalanced dosage sequences. Drug responsivity of both ADHD and TS symptoms will be assessed using direct observation procedures in the school and in the clinic playroom, laboratory performance tests, and clinician, teacher, and parent rating scales. A placebo controlled, double-blind event-related potential (ERP) study will be conducted immediately following the medication evaluation to examine drug related changes in brain function. Ongoing ERP studies in our laboratory will permit comparisons between the study sample and normal and ADHD children. The final phase of the proposed study is a six-month followup during which time ADHD and TS symptoms will be routinely assessed to determine changes over time.